1. Field of the Invention
The present invention relates to a thermal or heat-sensitive recording material for reflectgraphy which can be used to produce image copies and optical masks. In more detail, the present invention relates to a thermal recording material using a coagulatable proteinaceous material which becomes insoluble by application of heat energy and to a process for preparing a photomask using the same.
2. Description of the Prior Art
In the field of thermographic or thermosensitive recording process, many thermal recording materials have been known which provide relief images by imagewise or informationwise insolubilizing a recording layer by the application of a heat pattern and removing the soluble areas of the recording material. These thermal insolubilization type relief image-forming processes can roughly be classified into the following three categories.
Type A Process: Type A Process is a process in which a thermal record is formed upon direct application of light from a light source. In a recording layer formed in Type A Process heat is generated by a substance which converts light to heat (such as a dark blue or black pigment like carbon black, graphite, and oxides of certain heavy metals) upon exposure to light, thereby heat-insolubilizing the recording layer and thus forming a record.
Type B Process: Type B Process is a process in which a record cannot be formed upon the direct application of light alone but can be formed when direct light from a light source is combined with reflected light from an original. In Type B Process a threshold value of thermal change for recording is not reached upon application of direct light alone but is exceeded when light reflected form an original is added to the direct light. For exposure a recording layer in Type B Process is generally interposed between the light source and the original such that light passing through the recording layer combines with light reflected from the original to exceed the threshold value, thus resulting in heat insolubilization.
Type C Process: Type C Process is a process in which no heat change occurs in the recording layer by application of light from a light source, but heat generated and accumulated in a heat-absorbing region of an original by exposure is transferred to the recording layer (in most cases by thermal contact) thus producing imagewise heat insolubilization. Thus, recording is accomplished in this type process by heat conduction from the original.
Type A Process is of the direct exposure type, and Type B and C Processes are of the indirect exposure type, i.e., the reflection exposure type.
Type C Process is disclosed in, for example, Japanese Patent Publication No. 557/70 corresponding to British Pat. Nos. 1,139,891, 1,139,894, 1,139,895, and 1,139,896. The exposure time required for recording in Type C Process is generally sufficiently long that the heat transferred to the recording layer diffuses from the regions of the recording layer corresponding to the heat ray-absorbing regions of the original. Actually, electromagnetic radiation, for example, a heat ray or infrared radiation by filament tube is employed for this purpose. Thus, such results in some fringe insolubilization outside the areas to be recorded, and the images usually formed in the recording layer in Type C Process are not sharp and are of low quality.
Japanese Patent Publication No. 36080/71 corresponding to U.S. Pat. No. 3,661,579 discloses Type B Process and U.S. Pat. No. 3,580,719 discloses both Type A and B Processes in which a layer of a normally water-soluble polymer which when heated undergoes a loss in its normal solubility is contained, and a substance in heat-conducting relationship with the polymer which absorbs visible radiation and converts it into heat is contained in the recording layer. In reflectgraphy, when a xenon flash lamp having an exposure time of less than 10.sup.-2 second is used to expose the recording layer in Type B Process in contact with an original, the heat generated and accumulated in the light-absorbing regions of the original will not affect the recording member since the very short exposure prevents an accumulation of heat in these areas. Hence, recording in accordance with Type C Process, conduction, can not occur.
Japanese Patent Publication Nos. 9716/69, 9718/69, 22957/69, and 27919/71, corresponding to British Pat. No. 1,139,892, and U.S. Pat. Nos. 3,523,791, 3,793,025 and 3,628,953, respectively teach that Type B Process can be realized by reducing the exposure time (less than 10.sup.-2 second) to prevent the occurrence of recording in accordance with Type C Process and adjusting the optical density of the recording layer to not more than 1 and preferably to about 0.2 to 0.8 to prevent the occurrence of recording in accordance with Type A Process.
The limits placed on the construction of the recording material in conventional Type B and Type C Process as discussed above render them unsuitable for use in the production of photomasks. In Type C Process due to the long exposure times usually required and the heat diffusion which occurs during that time, high quality images having good delineation cannot be produced. Type B Process may not suffer from heat diffusion and may provide sharp images, however, the optical densities of the recording layer are not suitably high for use as photomasks.